1. Field of the Invention
The present invention relates to a conductive member, which is used in an image formation apparatus such as an electrophotographic copier, a laser printer, a facsimile and the like, and a process cartridge using the conductive member and an image formation apparatus using the process cartridge.
2. Description of the Prior Art
In a conventional electrophotographic image formation apparatus such as an electrophotographic copier, a laser printer, a facsimile and the like, a charge roller is generally used as a charge member for performing a charge processing to an image carrier or a photoreceptor. An explanatory view of the conventional electrophotographic image formation apparatus having the charge roller is illustrated in FIG. 4.
In FIG. 4, a numeral 120 represents the conventional electrophotographic image formation apparatus including an image carrier 101, a charge roller 102, a laser exposing device 103, a development roller 104, a power pack 105, a transfer roller 106, a cleaning device 108 and a surface potentiometer 109.
An electrostatic latent image is formed on a surface of the image carrier 101. The charge roller 102 contacts the image carrier 101 and performs the charge processing to the image carrier 101. The development roller 104 is used to have toners attracted to the electrostatic latent image on the surface of the image carrier 101 to form a toner image thereon. The power pack 105 is used to apply a DC voltage to the charge roller 102. The transfer roller 106 transfers the toner image on the surface of the image carrier 101 to a recording paper 107. The cleaning device 108 is for cleaning the image carrier 101 after the toner image is transferred. The surface potentiometer 109 is for determining a surface potential of the image carrier 101.
Moreover, the conventional electrophotographic image formation apparatus 120 is such an apparatus that a process cartridge thereof is detachable. In other words, the process cartridge 110 including in block four processing devices of the image carrier 101, the charge roller 102, the development roller 104 and the cleaning device 108 may be attached to the image formation apparatus or detached from it at will. It is also preferable for the process cartridge 110 to include at least the image carrier 101 and the charge roller 102. When the process cartridge 110 is attached to a predefined place of the image formation apparatus, it is connected to a driving system and an electric system on a main body of the image formation apparatus. Moreover, other functional units generally used in an electrographic processor are omitted in FIG. 3 since they are not necessary in the present invention.
A general image formation process via charging of the conventional electrophotographic image formation apparatus 120 will be explained as follows.
When a DC voltage is applied from the power pack 105 to the charge roller 102 contacting the image carrier 101, thus the surface of the image carrier 101 is charged uniformly with a high potential. It is known that such kind of charging mechanism which charges the surface of the image carrier 101 through the charge roller 102 follows the Paschen rule within a small space between the charge roller 102 and the image carrier 101. After the surface of the image carrier 101 is charged, an image light is projected by the exposing device 103 onto the surface of the image carrier 101, a potential of a portion wherever projected decreases.
Since the image light corresponds to the light amount distribution of the image, when the surface of the image carrier 101 is projected by the image light, a potential distribution corresponding to the image, in other words, an electrostatic latent image is formed thereon. When the portion of the surface of the image carrier 101 formed with the electrostatic latent image passes the development roller 104, the toner is attracted to the electrostatic latent image according to the potential levels and as a result there forms a visible toner image from the latent image.
The recording paper 107 is transported by a resist roller (not illustrated) at a predefined timing to the portion where the visible toner image is formed to overlap with the toner image. The recording paper 107 is peeled off from the image carrier 101 after the toner image is transferred by the transfer roller 106 onto the recording paper 107. The peeled recording paper 107 is transported through a transportation path to a fuser unit (not illustrated) to be fused via heating and finally expelled out from the apparatus 120. After that, any remaining toner on the surface of the image carrier 101 is removed by the cleaning device 108, and any remaining charge thereon is discharged by a quenching lamp (not illustrated). Thus, the apparatus 120 is ready for a next image formation process.
In general, a charge method using a charge roller is to charge the image carrier by contacting the charge roller with the image carrier. For such charge method by contact, there are such problems as listed in the following:
(1) A material constituting the charge roller exudes out from the charge roller and thus leaves a trail when it contacts and moves on a surface of a member to be charged;
(2) When an alternating currency is applied to the charge roller, the charge roller contacting the member to be charged vibrates, causing a charging noise;
(3) Adhesion of toners from the image carrier to the charge roller (In particular such adhesion occurs more easily when there is an exudation as mentioned above) lowers charging performance of the charge roller;
(4) A material constituting the charge roller adheres to the image carrier; and
(5) The charge roller deforms permanently when the image carrier has been idle for a long time.
To solve such problems, an adjacent charge method by making the charge roller close to the image carrier has been disclosed in Japan Patents Laid-Open Nos. H3-240076, H4-358175 and H5-107871. Such adjacent charge method performs charging to the image carrier by applying a voltage to the charge roller which is disposed oppositely to the image carrier at a closet distance from 50 μm to 100 μm. Since the charge roller and the image carrier are not contacted in the adjacent charge method, it will not have such problems as those in the conventional charge method by contact and will not have the problem such as that the charging performance of the charge roller is lowered by the adhesion of toners to the charge roller.
Characteristic properties required for the charge roller used in the adjacent charge method are different to those required for the charge roller used in the charge method by contact. Generally the charge roller used in the charge method by contact is formed by coating an elastic member such as a vulcanized rubber or the like around a cored bar. In order to charge the image carrier uniformly using such charge roller, it is mandatory that the charge roller contact uniformly with the image carrier.
However, in a case where the charge roller formed from an elastic member such as a vulcanized rubber or the like is used in the adjacent charge method, there are such problems as listed in the following:
(1) It is necessary to dispose a gap preserving member such as a spacer or the like at both ends of the charge roller corresponding to none image areas in order to provide a gap between the charge roller and the image carrier. While it is difficult for the gap to be kept uniformly because of the deformation of the charge roller formed from the elastic member, and this causes potential variations and image irregularities resulted from the potential variations.
(2) It is easy for the vulcanized rubber constituting the elastic member to have strain and deformation over time, and as a result the gap will vary over time.
To solve such problems it has been proposed to use a non-elastic member, a thermoplastic resin which makes it possible to uniform the gap between the image carrier and the charge roller. It is known that a charging mechanism which charges the surface of the image carrier (photoreceptor drum) through the charge roller follows the Paschen rule within a small space between the charge roller and the image carrier. In order to keep the image carrier at a predefined charge potential level, it is necessary to control the electric resistance value of the thermoplastic resin within a semi conductive range of about 106 to 109 Ωcm.
Among methods to control the electric resistance value, there is one to disperse conductive pigments such as carbon blacks or the like in the thermoplastic resin. However, such method will cause bigger irregular variations on the electric resistance value, resulting in a partially unfavorable charging which leads to a problem of improper image formation.
There is also another method to control the electric resistance value of an electric resistance adjusting layer is to add an ion conductive material, in other words a electrolyte salt such as a lithium salt or the like to the electric resistance adjusting layer. Such ion conductive material may be dispersed at a molecular level in a matrix resin, therefore the irregular variations on the electric resistance value is smaller than that dispersed with the conductive pigments, resulting in a smaller partially unfavorable charging which will not affect the image quality. However, the electrolyte salt such as the lithium salt or the like has a low molecular weight and thus has a character to bleed out to the surface of the matrix resin easily. When the electrolyte salt bleeds out to the surface of the charge roller, it will attract toners, leading to a problem of improper image formation.
In order to avoid the bleeding out of the electrolyte salt, it has been proposed to use a high molecular ion conductive material which is dispersed and fixed in the matrix resin. In such case, it is difficult for the high molecular ion conductive material to bleed out to the surface of the matrix resin. Japan Patent Laid-Open No. H7-121009 discloses a charge roller which includes an electric resistance adjusting layer made from the matrix resin by dispersing and fixing therein the high molecular ion conductive material having a quaternary ammonium group and has fewer bleeding out over time.
However it is impossible to control the electric resistance value within the semi conductive range only by using the high molecular ion conductive material, other methods are necessarily needed to regulate the electric resistance value.
In such dispersion system of the high molecular ion conductive material, in a case that the high molecular ion conductive material is an island ingredient of a sea-island dispersion, the isolating matrix resin retards a currency therethrough, thus problems such as the electric resistance value for the electric resistance adjusting layer decreases below the semi conductive range or the electric resistance value depends much heavily on a power voltage arise. Moreover, when a dispersed particle of the sea-island dispersion becomes large in diameter, there is such problem that the electric resistance value varies as strength of a weld portion formed in molding decreases.
Furthermore, when resins with low mechanical strength or resin with bad compatibility are used as the matrix resin, cracks may occur to the weld portion of the electric resistance adjusting layer according to an electric or mechanical stress during using, or to a volume variation caused by time or circumstance. The electric resistance variation of the weld portion may cause a problem of partially improper images.